I am going to guess the Cor.
I have been holding back my purchase on a DC pump for it to come out. It's getting frustrating on the wait, as the rumor release date keeps passing by. Last one was "this is a 2016 product" which was said by Neptune. There are two reason why I am not considering M1. One, I have heard they have a lot heat issues. Two, I refuse to buy from any company that blocks out Apex on purpose. The reason I want to control it, what if I am on vacation and the house sitter calls me and says something going on. After figuring out the problem, the temporary solution is to dial back the pump until I can get to it. Yes, an unlikely scenario, but I want that option.
We should be thankful that Neptune is taking their time releasing the COR. Nothing good happens fast in this hobby!
I personally haven't seen many problems with the M1. I have seen some for the L1 though. Most of those heat issues seem to be due the the particular application that Ecotech has been unable to replicate. Hopefully they will figure it out.
As for the ecotech not being compatible with the apex. They have their reasons and that is their choice. Personally I would never put a return pump on my controller. To vital part of the equation to purposely put in additional possible points of failure.